The present invention relates to digital magnetic recording/reproducing apparatuses such as magnetic disk recorders, and particularly to a signal processing system and apparatus for recording digital data at high density.
The demand for high-density recording and fast processing in magnetic disk recorders has more and more escalated. The signal processing techniques in the signal recording and reproducing system to support these requirements have also been developed towards high-density recording and fast processing. As to the recording code, the coding rate R has been increased, and now the main coding rate R in current use is R=8/9. In addition, recently a higher rate recording code of R=16/17 has started to be practically used. Moreover, a partial response equalization system is employed in order to prevent the signal-to-noise ratio from being reduced by the increase of the intercode interference involved with the high-density recording. PR4ML (Partial Response Class 4 with Maximum Likelihood Detection) is utilized to detect a signal sequence nearest to the reproduced signal by the Viterbi algorithm (best likelihood sequence estimation) using a known interference occurring in the reproducing channel. A device for this purpose is already incorporated as an LSI (Large Scale Integrated Circuit) in a magnetic disk product. It is known that if the input signal sequence to the PR4ML processor is binary data of 0, 1, the minimum squared Euclidean distance (MSED) between signal sequences produced by PR4ML is 2. Therefore, PR4ML is improved 3 dB in its tolerance to noise over the peak detection system (MSED=1) for deciding magnetic information by only 0, 1 without the best likelihood sequence estimation.
In order to achieve higher-density recording than PR4ML, it is necessary to use such a signal processing technique as to increase the MSED. For this purpose, there are EPR4ML (Extended PR4ML), and EEPR4ML (Extended EPR4ML). These are the extensions of the PR4ML idea. The values of MSED for those extensions are known to be 4, 6 (binary conversion), respectively. In addition, the channel state number is 8 for EPR4ML, and 16 for EEPR4ML.
FIG. 2 is a block diagram of a conventional digital magnetic recording/reproducing apparatus. As illustrated, on the recording side, an information sequence of “0”s and “1”s as digital data is converted into a high rate code such as R=8/9 or 16/17 by a recording coder 201. The recording code, as well known, has a limited number of successive “0”s provided to prevent the timing extraction and gain control (not shown in FIG. 2) in the reproducing section from being reduced in their performances. The recording coded sequence is further supplied to a precoder 202, thereby being converted into a code in which 1/(1+D) is treated as a transfer function. Only when data of “1” is supplied to the precoder 202, the output is changed from just the preceding value. Here, D is the delay operator, and the delay time is equal to the bit distance. The precoder 202 is able to suppress the decoded error propagation length after the Viterbi detection on the reproducing section to a limited value. The precoded sequence is supplied through an amplifier 203 to a record head 204, by which it is recorded on a magnetic recording medium 205 as magnetic information.
On the reproducing side, the magnetic information recorded on the magnetic recording medium 205 is reproduced by a reproduce head 206, and supplied to an amplifier 207, thereby being converted into an analog electric signal. This signal is supplied to an A/D converter 208, which samples it at each bit interval, thereby converting it into a digital signal. The digital signal is fed to a PR equalization circuit 209, which then equalizes it into partial response channels such as PR4, EPR4, and EEPR4. The PR equalization can be easily realized by a well known transversal filter. The output from the PR equalization circuit 209 contains noise added up to a signal level determined by the PR channel characteristic. Here, the noise contains the medium noise, the noise mixed from the reproduce-side head, and the noise caused by A/D quantization. These noises, when passed through the PR equalization circuit 209, become colored noises with a correlation. The equalized signal with the noise added is supplied to a Viterbi detector 210 that performs MLSE (Maximum Likelihood Sequence Estimation). Thus, it produces the most probable data sequence, or a data sequence most likely to resemble the input signal sequence. Since the reverse characteristic (1+D) to the precoder 202 can be produced as NRZI (Non Return to Zero Inverted) within the Viterbi detector as well known, a postcoder can be removed. The data sequence from the Viterbi detector 210 is decoded into the information sequence by a recording decoder 211.
Thus, this conventional digital magnetic recording/reproducing apparatus employs a high-rate recording code, and combines the partial response and Viterbi detection, thereby increasing the signal-to-noise ratio of the reproduced signal for high-density recording.
In recent years, other various signal processing systems for higher-density recording have been discussed in addition to the above-described conventional apparatus. As a powerful one of those systems, there is a List Viterbi Algorithm (hereinafter referred to as an LVA). In this system, the Viterbi detector 210 detects the most reliable data sequence (best sequence), the second most reliable data sequence (2nd best sequence), the third most reliable data sequence (3rd best sequence), . . . , and the nth most reliable data sequence (nth best sequence), produces these as proposed sequences, or candidates, detects a decoding error of each candidate by use of CRC (Cyclic Redundancy Check) or the like, and generates a decoded output of candidates with no decoding error. The LVA is able to greatly improve the decoding error characteristic of the Viterbi detector. The details of the LVA are described in N. Seshadri et al., “List Viterbi Decoding Algorithms with Applications”, IEEE Transactions on Communications, Vol. 42, No. 2/3/4, February/March/April 1994, pp. 313-323. The LVA was originally contrived for application to a communications field such as mobile radio communication, but it has not yet been applied to a magnetic recording/reproducing apparatus. The present invention fundamentally applies the LVA to a magnetic recording/reproducing apparatus as will be described later with respect to various embodiments. However, a simple application of the conventional idea directly to the apparatus cannot achieve high-density recording. This will be described in detail later.
The basic idea of the LVA will be described using as an example an EPR4 channel with reference to the trellis diagram of FIG. 3. FIG. 3 shows the structure of the EPR4 channel with respect to time (here, time 0 to 8). The number of channel states is 8. The states 000, 111 are expressed as 0, . . . , 7, respectively. When −1 and +1 (which correspond to binary data 0, 1, respectively) are applied to the respective states, the associated PR equalization signals (expressed in binary conversion values as illustrated) are produced, making the channel states transitional.
It is now assumed that the state at time 0 is 1. When data 1 is applied at time 1, the PR equalization output of 2 is produced, and thus the channel state is shifted from 1 to 3.
The equalization signal of an EPR4 channel takes five values of 2, 1, 0, −1, and −2. In FIG. 3, it is assumed that the line A indicates the state transition sequence (path) corresponding to the correct data sequence. However, as a result of the Viterbi detection, the decoded signal may sometimes be in error as indicated by line B. As will be seen from the figure, the squared Euclidean distance is 4 between the correct decoded path (the equalization signal value sequence is 0, 0, 0, 1, 1, −1, 0, 1) and the erroneous decoded path (the equalization signal value sequence is 0, 0, 1, 2, 1, −1, −1, 0). This is an error event of MSED=4. In general, the decoding error characteristic of the maximum likelihood sequence estimation is determined by MSED. The error event of MSED=4 occurs at the highest frequency in EPR4ML.
On the other hand, the LVA estimates and produces a plurality of candidates (2nd best, 3rd best, . . . , nth best sequences) including the maximum likelihood estimated sequence (best sequence) in the order of higher probability (likelihood ratio). If there are two candidates, the path (correct sequence) indicated by the line A is produced as the best sequence, and the path (error sequence) indicated by the line B is produced as the 2nd best sequence as shown in FIG. 3. Since these candidates include the correct decoded path, the correct one can be selected by CRC error detection to both sequences. Therefore, the LVA can remove the error event of MSED=4, and a larger Euclidean distance error event will occur. In other words, the MSED is equivalently expanded, and thus the decoded error characteristic can be improved. In the case of EPR4ML, the MSED is expanded to 6 (an example of the error event of MSED=6 is indicated by line C). This means that the S/N (signal-to-noise ratio) of the reproduced signal is improved about 1.8 dB. Although an EPR4 channel has here been given as an example, the LVA can be applied to other PR channels than an EPR4 channel, and hence the decoded error characteristics can be improved.
The LVA is classified into a serial system and a parallel system as described in the above-given document. The serial system, at the time of the Viterbi detection, decides the 2nd best sequence if the best sequence becomes erroneous as the candidate, the 3rd best sequence if the 2nd best sequence is erroneous, and similarly the nth best sequence if the (n−1)th best sequence is erroneous. Thus, the LVA repeats the sequence estimation processing until there is no error. Of course, since n is practically definite, when error is still present after n repetitions, a signal indicating the presence of decoding error is transmitted to the transmission side, requesting retransmission.
The parallel system, at the time of Viterbi detection, produces n candidates at a time, and selects one with no error decided by CRC. In this case, too, when error is detected in all candidates, the signal indicating the presence of decoding error is transmitted to the transmission side, requesting retransmission.
When the LVA is applied to a magnetic recording/reproducing apparatus, the serial system is able to achieve LVA detection by a relatively simple construction, but has too large a maximum processing delay (n times as large as in the prior art) to be applied to fast processing. The parallel system needs a large amount of ACS (Add, Compare, Select) operations (n times as much as in the prior art) for estimating a proposed candidate, but can perform fast processing since candidates can be produced in parallel. Therefore, the parallel system capable of fast processing is suited to the magnetic recording. The increase of the circuit scale causes practically no important problem since the process for LSIs (Large Scale Integrated Circuits) has recently fast developed.
According to the above document, the LVA of the parallel system periodically fixes both of the starting and ending states to one (in the other cases than these states, N states) on the trellis diagram as shown in FIG. 4. On the starting side, the likelihood ratios of all candidates are reinitialized to the same value as the likelihood ratio of the best sequence. This is because the LVA needs to select n candidates having different likelihood ratios to be estimated within an interval of M bits, that is, to always choose n different candidates of unlike likelihood ratios within this interval. Therefore, the n candidates to be detected at intervals of M bits each are all sure to be different.
In this system, if the path memory length necessary for LVA detection is represented by K bits and the block length of a CRC code by L bits, then the relation of K≧L must be satisfied, that is, the necessary path memory length is required to be set longer than or equal to the CRC block length. The reason for this will be explained with reference to FIG. 5. FIG. 5 shows the relations between the LVA path memory length and the CRC block length. The number of candidates is 2. Here, it is assumed that the S/N ratio is so high as to consider a probability that two or more error events will occur to be negligibly small. When the path memory length is larger than the CRC block length, the 2nd best sequence covers, even if an error event occurs in the first block of the best sequence, that error as shown in FIG. 5 at (a), that is, a correct sequence is included in the first or second block of either the best or 2nd best sequence. However, when the path memory length is smaller than the CRC block length, errors can arise in both sequences over a CRC block length as shown in FIG. 5 at (b). This phenomenon in the prior art is caused because a different sequence is always selected for the LVA at each path memory length. Thus, the LVA must be matched to the CRC block length, and the necessary path memory length must be set to be longer than or equal to the CRC block length.
When the LVA is applied to a magnetic recording/reproducing apparatus, the following problem is caused.
(1) Since the necessary path memory length for the LVA detector is required to be larger than the CRC block length, making the CRC block length long (more than about 100 bits) makes it impractical to construct the LVA detector. In the conventional system, the LVA is utilized in a communications field such as mobile radio communication, and thus it is not necessary to extremely increase the coding rate. Therefore, the CRC block length is as short as at most 10 bits (the path memory length is about 20 bits), and the coding rate may be about 8/10. In practice, however, since the conventional system employs a combination of a convolution code of rate 1/2 as a code for constructing the trellis diagram in addition to the CRC code, the coding rate of the whole system is. substantially as fairly low as 4/10. For the magnetic recording, the coding rate of the whole system is required to be 8/9 or more for high-density recording. Thus, the CRC block length for this purpose becomes very long (more than about 100 bits), and thus the LVA detector that needs a path memory length longer than that cannot be practically constructed since the processing delay becomes remarkably large.
(2) In the conventional system, each of the starting and ending state sides is periodically fixed to one state on the trellis diagram, and the likelihood ratios of all candidates on the starting state side are initialized to be the likelihood ratio of the best sequence. Thus, different candidates are always obtained at each path memory length. Therefore, the number of codes that can be assigned on the trellis is considerably limited, and thus the possible recording code rate is low (about 1/2) on the trellis. In other words, the conventional LVA cannot achieve more than the current rate 8/9.
Under the presence of the above problems, the conventional LVA cannot accomplish high-density recording.